Bekhorot 17B

וחייב במתנות, ורבי יוסי פוטר. מת אחד מהן ⁠— רבי טרפון אומר: יחלוקו, רבי עקיבא אומר: המוציא מחבירו עליו הראיה. זכר ונקבה ⁠— אין כאן לכהן כלום.

And when he slaughters the animal he is obligated to have the gifts of the priesthood taken from it, i.e., the gifts that one is required to give a priest from a non-sacred animal: The foreleg, the jaw, and the maw. And R' Yosei deems him exempt from giving those gifts. If one of the two born together died, R' Tarfon says: The priest and the owner divide the remaining lamb. R' Akiva says: Since there is uncertainty to whom it belongs, it remains in the possession of the owner, as the burden of proof rests upon the claimant. If a male and a female offspring were born together, everyone agrees that the priest has nothing here.

גמ׳ אמרי דבי רבי ינאי: לרבי יוסי הגלילי שמעינן ליה דאמר אפשר לצמצם בידי שמים, וכל שכן בידי אדם, ורבנן ⁠— בידי שמים אי אפשר לצמצם, בידי אדם מאי?

Talmud: The Mishnah teaches that the Rabbis disagree with R' Yosei HaGelili, maintaining that it is impossible for the time of twins’ births to coincide precisely. They said in the school of R' Yannai: According to R' Yosei HaGelili, we heard in the Mishnah that he says: It is possible for two events that are in the hands of Heaven to coincide precisely, and all the more so it is clear that it is possible for two events that are in human hands to coincide precisely, as they are intended to be simultaneous. But according to the Rabbis, evidently they maintain that it is impossible for two matters that are in the hands of Heaven to coincide precisely, but with regard to events that are in human hands, what is their opinion?

תא שמע: חוט של סיקרא חוגרו באמצע, להבדיל בין דמים העליונים ובין דמים התחתונים. ואי אמרת אי אפשר לצמצם בידי אדם, זימנין דקא יהיב עליונים למטה ותחתונים למעלה! דמרווח בה פורתא.

The Talmud suggests: Come and hear a proof from a Mishnah (Middot 3:1): A red line encircled the altar in the middle, to separate between the blood that must be presented on the upper part of the altar and the blood that must be presented on the lower section of the altar. And if you say that it is impossible for two matters that are in human hands to coincide precisely, i.e., that people cannot measure the precise midpoint of an area, then that line will not be precisely in the middle, which means that there will be times when the priest puts blood meant for the upper part of the altar upon the lower section, and blood meant for the lower section above. The Talmud rejects this proof: In that case, one widens the line a little so that it certainly covers the middle part of the altar.